Disorders associated with obesity represent some of the biggest medical challenges of this century. These disorders encompass hypertriglyceridemia, cardiovascular disease, nonalcoholic fatty liver disease (NAFLD), and diabetes. We have shown in animal models with insulin resistance that hypertriglyceridemia and NAFLD share the same underlying molecular alteration: elevated nuclear SREBP-1c, which leads to increased synthesis of fatty acids and triglycerides in the liver. Two SREBP isoforms exist in most organs, SREBP-1c and SREBP-2. Remarkably, feedback inhibition of cholesterol synthesis and feedback inhibition of fatty acid synthesis are mediated by inhibition of the proteolytic processing of SREBP-2 and SREBP-1c, respectively. Here, our goal is to delineate the molecular mechanism by which polyunsaturated fatty acids (PUFAs) mediate the feedback suppression of fatty acid synthesis by inhibiting the activation of SREBP-1. AIM 1 will examine a fundamental unanswered question in regulation of lipogenesis, namely, how is SREBP-1c processing regulated independently from SREBP-2 despite using the same molecular machinery? These studies will define the specific region of SREBP-1 that is responsible for mediating PUFA inhibition of SREBP-1 cleavage and activation. We will use biochemical purification and somatic cell genetics to identify a putative protein that binds PUFAs and interacts with SREBP-1, but not SREBP-2, to prevent cleavage and activation. We will also define how PUFAs alter the membrane lipid composition of the endoplasmic reticulum and how these changes prevent SREBP-1 cleavage. AIM 2 will investigate a recently discovered protein, Ubxd8, that binds and senses unsaturated fatty acids. We will determine whether PUFAs inactivate Ubxd8 in mouse livers and elucidate its role in regulating SREBP-1c processing in vivo. AIM 3 will explore why inhibiting the first committed enzyme in fatty acid synthesis (acetyl-CoA carboxylase) prevents the development of fatty liver, even though this inhibition leads to activation of SREBP- 1c and hypertriglyceridemia. At its completion, RP 3 will elucidate new mechanisms that regulate SREBP activity and fatty acid synthesis in liver, thereby providing new therapeutic opportunities for treatment of hypertriglyceridemia and NAFLD. C/PPG 2015 ? RP3 ? 30-line Summary